|
| Aurora Desktop, Windows Vista Build 5270 | | | |
Aurora is a wallpaper created by the Microsoft design team, set as the default Windows Vista wallpaper (except on Starter & Entrerprise editions).
Auroras are natural colored light displays, which are usually observed in the night sky, particularly in the polar zone. Some scientists therefore call them "polar auroras" (or "aurorae polaris").
In northern latitudes, it is known as the aurora borealis (IPA /ɔˈɹɔɹə bɔɹiˈælɪs/), named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas. Especially in Europe, it often appears as a reddish glow on the northern horizon, as if the sun were rising from an unusual direction. The aurora borealis is also called the northern lights, as it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April.
Its southern counterpart, aurora australis, has similar properties. Australis is the Latin word for "of the South".
Auroras are now known to be caused by the collision of charged particles (ions (+) (-) ) found in the magnetosphere, with atoms in the Earth's upper atmosphere (at altitudes above 80 km). These particles travel into space with speeds of 300 to 1200 kilometers per second. A cloud of these particles is called plasma, and a stream of plasma coming from the sun is called solar wind. These charged particles are typically energized to levels between 1 and 15 keV and, as they collide with atoms of gases in the atmosphere, the atoms become excited. Shortly afterwards, the atoms emit their gained energy as light (see Fluorescence). Light emitted by the Aurora tends to be dominated by emissions from atomic oxygen, resulting in a greenish glow (at a wavelength of 557.7 nm) and - especially at lower energy levels and at higher altitudes - the dark-red glow (at 630.0 nm of wavelength). Both of these represent forbidden transitions of electrons of atomic oxygen that, in absence of newer collisions, persist for a long time and account for the slow brightening and fading (0.5-1 s) of auroral rays. Many other colors - especially those emitted by atomic and molecular nitrogen (blue and purple, respectively) - can also be observed. These, however, vary much faster and reveal the true dynamic nature of auroras.
As well as visible light, aurorae emit infrared (NIR and IR) and ultraviolet (UV) rays as well as X-rays (e.g. as observed by the Polar spacecraft). While the visible light emissions of aurorae can easily be seen on Earth, the UV and X-ray emissions are best seen from space, as the Earth's atmosphere tends to absorb and attenuate these emissions. Wikipedia |
|
|
